1. Field of the Invention
This invention relates to a portable medium such as a multifuctional integrated circuit (IC) card incorporating a central processor unit (CPU), a data memory, a battery and so on, which is used for calculation and timekeeping, and adapted for use with terminals.
2. Description of the Related Art
Recently, multifuctional IC cards have been developed which incorporate a CPU, a data memory, a battery and so on. The IC card itself serves as a hand-held calculator or timepiece with the aid of a keyboard and display device mounted on the top thereof. Such an IC card is disclosed in U.S. Pat. No. 4,697,072 (kawana), for instance. However, it does not mention the prevention of a built-in battery of the card from being excessively consumed.
Where the IC card is used along, or used in off-line operations, data processing is performed by the use of a built-in battery. To decrease power dissipation, therefore, low frequency clocks (low speed clocks) are used for CPU clocks.
However, since the low frequency clocks result in low processing speed, high frequency clocks (high speed clocks) are sometimes used to perform the data processing faster. Where the high frequency clocks are used, the power dissipation increases in the CPU so that the life of the battery will decrease.